1. Field of the Invention
The present invention relates in general to a hydraulic priority circuit and more specifically to a hydraulic priority circuit having a single pump and a means for varying the output of the pump as a function of changing demands for fluid flow in a primary work system and in a secondary work system.
2. Description of the Prior Art
The use of a priority flow control valve in a fork lift truck, for example, to combine a steering circuit and a secondary circuit including fork tilt and lift circuits is well-known in the art. In such systems a priority valve commonly assures that a full flow of fluid is continuously supplied to the steering circuit to assure steering control independent of other hydraulic system requirements. Obviously, such a system wastes a significant amount of flow power which might otherwise be available to the rest of the hydraulic system. More recent hydraulic circuits have included priority control valve means wherein a lesser control fluid flow is provided to the steering circuit, with the full flow only being provided upon demand to the steering circuit. However even in such a system as this, the pump is continuously operated at a full flow rate so that waste of system power still occurs since the excess flow is also not necessarily needed by the hydraulic system.